Polyethylene is the most widely used thermoplastic polymer in the world, being made into products ranging from clear food wrap and plastic bags to laundry detergent bottles and automobile fuel tanks. Among the different polyethylene types, linear low density polyethylene (LLDPE) represents almost 30% of the total polyethylene, and the development of new catalysts and process technologies has motivated continuous improvement of its properties and the ability to tailor it for a wide range of applications. These resins represent a considerable advance in physical properties over the long chain branched low density polyethylenes (LDPEs), made by conventional high pressure process.
It is well known that polyethylene may be prepared by the polymerization of ethylene (optionally with one or more alpha-olefins such as butene-1, hexene-1, octene-1, etc) using coordination catalysts, such as Ziegler-Natta systems which comprises a transition metal compound. Polyethylene produced in this manner, particularly LLDPE, is in widespread commercial use. However, this conventional LLDPE, suffers from a number of deficiencies such as, high amount of low molecular weight and also large amount of very high molecular weight homopolymer of ethylene. The heterogeneous nature of these polymers generally detracts from the physical properties made from them. A great deal of effort has been directed towards the preparation of more homogeneous LLDPE resins, which mitigate this problem, such as the development of catalytic systems, such as metallocenes or post-metallocenes, with better comonomer responses (alpha-olefins) and producing less wax content.
The molecular weight distribution of a polymer is a very important factor in determining its mechanical properties and processing behavior. It is generally found that the mechanical properties of a polymer deteriorate with broadening MWD. Polymers with narrower MWD crystallize more uniformly and exhibit better physical properties such as increased dimensional stability, higher impact resistance, greater toughness at low temperatures and higher resistance to environmental stress cracking. However, broadening of the MWD enhances the polymer processability. In an extrusion process, resins with broader MWDs exhibit lower viscosities at processing shear rates. One method of tailoring the property-processing relationship is to control the shape of the MWD. For many industrial resins, especially for pipe grade PEs, the MWDs are very broad and sometimes bimodal. Bimodal resins have a high molecular weight component to impart strength and toughness and a low molecular weight component to facilitate extrusion.
The comonomer distribution is also an important property that can be tailored for the desired application. For Ziegler-Natta LLDPE, it is generally found that the comonomer distribution is broad and uneven. A greater concentration of comonomer is found on the shorter chains. Single-site LLDPE, such as LLDPE produced by metallocene catalysts, has a narrow and more uniform distribution of comonomer. A recent trend is to produce resins with ‘reverse comonomer distributions’ in which the comonomer is placed on the high molecular weight chains.
Blending of two or more polymers is often applied to control the overall property profile of materials and/or to aim for cost reduction. For polyethylene resins, blending primarily aims at improving the balance of processability and mechanical properties of the final product. Important industrial applications comprise: blending of polyethylenes with different molecular weights, thus achieving broad molecular weight distributions, and blending of homo- and copolymers with different degrees of branching and branching types.
Blending can be achieved by: post-polymerization mixing of the different polymers, e.g., in an extruder, or, more conveniently, by directly generating the different polymers during the polymerization process (‘reactor blending’). The latter technique can employ multiple reactors, generating different polymers by varying the reaction conditions in each reactor, or within a single reactor two or more polymerization catalysts can be used. For metallocene or Ziegler-Natta catalysts, reactor blending is well established and is applied on an industrial scale.